Patients with von Willebrand's disease (vWD) have a deficiency in factor VIII procoagulant, immunologic, and ristocetin activity. A new antigen, vWagII, has been found to also be deficient in patients with severe vWD but not decreased in mild vWD. Further studies will be undertaken to characterize this antigen which appears to circulate in plasma and is platelet related. Crossed immunoelectrophoresis, SDS polyacrylamide gels, sucrose gradient ultracentrifugation will be used to study vWagII. The immunologic relationship to proteolytic fragments of plasma and platelet factor VIII will be studied by differential absorption using the above techniques. Since vWagII appears to be released from platelets, the various platelet components, cytosol, granules, and membrane, will be separated to attempt localization of vWagII within the platelet.